Being in recovery from Alcoholism for many years now I can tell you, that:
I do not attempt to talk to a former 'sober' friend while they are drinking, I will only be talking to the bottle.
I tell them to give me a call when they are sober if they would like to talk.
And I walk away.
Then as I walk away I say a prayer for them.
It is sad when this happens, but it does happen, more than most of us care to know. The person who has had some recovery, KNOWS where to go to get help, I cannot help them. I cannot fix them. and I sure as hell can't cure them.
Detachment can be very hard.
Best of luck to you and your friend.
Love and hugs,